Root Border-Like Cells of Arabidopsis. Microscopical Characterization and Role in the Interaction with Rhizobacteria  

Abstract

Plant roots of many species produce thousands of cells that are released daily into the rhizosphere. These cells are commonly termed border cells because of their major role in constituting a biotic boundary layer between the root surface and the soil. In this study, we investigated the occurrence and ultrastructure of such cells in Arabidopsis (Arabidopsis thaliana) using light and electron microscopy coupled to high-pressure freezing. The secretion of cell wall molecules including pectic polysaccharides and arabinogalactan-proteins (AGPs) was examined also using immunofluorescence microscopy and a set of anticarbohydrate antibodies. We show that root tips of Arabidopsis seedlings released cell layers in an organized pattern that differs from the rather randomly dispersed release observed in other plant species studied to date. Therefore, we termed such cells border-like cells (BLC). Electron microscopical results revealed that BLC are rich in mitochondria, Golgi stacks, and Golgi-derived vesicles, suggesting that these cells are actively engaged in secretion of materials to their cell walls. Immunocytochemical data demonstrated that pectins as well as AGPs are among secreted material as revealed by the high level of expression of AGPepitopes. In particular, the JIM13-AGP epitope was found exclusively associated with BLC and peripheral cells in the root cap region. In addition, we investigated the function of BLC and root cap cell AGPs in the interaction with rhizobacteria using AGP-disrupting agents and a strain of Rhizobium sp. expressing a green fluorescent protein. Our findings demonstrate that alteration of AGPs significantly inhibits the attachment of the bacteria to the surface of BLC and root tip.Many plants can produce large numbers of metabolically active root ''border'' cells that are programmed to separate from each other and to be released from the root tip periphery into the external environment (Brigham et al., 1995a;Hawes et al., 1998). Root border cells are defined as cells that are released into solution within seconds when root tips are placed into water (Hawes et al., 2000(Hawes et al., , 2003. In the absence of free water, these cells remain adherent to the root tips. The border cells of most species remain viable even after separation from the root tips (Hawes et al., 2000) and can survive independently from roots in vitro as well as in natural field conditions. Interestingly, border cells can undergo cell division in vitro and develop into callus tissue (Hawes and Lin, 1990).As to the role of the border cells, Brigham et al. (1995b) have proposed that these cells are differentiated tissue of the root system that modulates the environment of the plant root by producing specific substances to be released into the rhizosphere. Proteins synthesized in border cells exhibit profiles that are distinct, qualitatively and quantitatively, from those synthesized in the root tips (Brigham et al., 1995b). These important differences in protein expression profiles are correlated with similarly distinct...

The Organization Pattern of Root Border-Like Cells of Arabidopsis Is Dependent on Cell Wall Homogalacturonan

“…It has also been suggested that species of the Brassicaceae family including Arabidopsis (Arabidopsis thaliana) do not produce border cells (Hawes et al, 2003). Indeed, the Arabidopsis root tip does not produce isolated border cells per se, but it does produce and release cells that remain attached to each other, forming a block of several cell layers called border-like cells (Vicré et al, 2005;Fig. 1).…”

“…they do not disperse individually) suggests that they might have a specific cell wall composition and/or structure that makes them resistant to cell wall-hydrolyzing enzymes or that the enzymes are not present or not functional . The only information on cell wall composition of Arabidopsis border-like cells was obtained from immunocytochemical studies, in which it has been shown that the cell wall of border-like cells is rich in pectic homogalacturonan and arabinogalactanproteins, two wall polymers believed to be involved in cell adhesion in plants (Vicré et al, 2005). Based on this observation, we postulated that pectic polysaccharides of the cell wall may serve as a glue to cement border-like cells together, leading to that particular organization (Vicré et al, 2005).…”

Root exudate ofSolanum tuberosumis enriched in galactose-containing molecules and impacts the growth ofPectobacterium atrosepticum

“…Our data also show the occurrence of a thick mucilage that surrounds border cells. In plants, root mucilage is mainly produced and secreted by the root cap and border cells (Moody et al, 1988;Vicré et al, 2005;Ma et al, 2010). Carbohydrates have been shown to be major components of root mucilage in many plant species, including cowpea, pea, wheat and maize (Moody et al, 1988;Cannesan et al, 2011).…”

“…Roots were carefully washed in deionized water and observed using a bright-field microscope (Durand et al, 2009). To test viability, staining with 5 mM calcein acetoxymethyl ester (calcein-AM; Sigma) was performed as described in Vicré et al (2005). Briefly, roots and root border cells were stained for 1 h, carefully washed in deonized water and observed using a microscope equipped with UV fluorescence (excitation filter, 490 nm; emission filter, 520 nm).…”

“…Roots from 10-d-old germs were fixed for 60 min in 1 % glutaraldehyde, 4 % paraformaldehyde in 50 mM 1,4-piperazinediethanesulfonic acid (PIPES) and 1 mM CaCl 2 , pH 7, and immunolabelled according to Willats et al (2001). Roots were washed in phosphate-buffered saline (PBS) containing 1 % (w/v) bovine serum albumin (BSA) and then incubated overnight in JIM13 (1:50), LM1 (1:50) or LM2 (1:50), diluted in PBS-1 % BSA containing 1:30 normal goat serum, as described previously by Vicré et al (2005). Roots were carefully washed and incubated with anti-rat immunoglobulin G (IgG) (dilution 1:5000) coupled to fluorescein isothiocyanate (Sigma).…”